Solid-state imaging device and camera

ABSTRACT

A solid-state imaging device which includes a plurality of pixels in an arrangement, each of the pixels including a photoelectric conversion element, pixel transistors including a transfer transistor, and a floating diffusion region, in which the channel width of transfer gate of the transfer transistor is formed to be larger on a side of the floating diffusion region than on a side of the photoelectric conversion element.

CROSS REFERENCES TO RELATED APPLICATIONS

The application is a continuation of co-pending U.S. application Ser.No. 12/275,489, filed on Nov. 21, 2008, which is incorporated herein byreference to the extent permitted by law. The present invention claimspriority to Japanese Patent Application No. JP 2007-311183 filed in theJapanese Patent Office on Nov. 30, 2007, the entire contents of whichbeing incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention generally relates to solid-state imaging devices andcameras. More particularly, the invention relates to a solid-stateimaging device and a camera provided with the solid-state imagingdevice.

2. Description of the Related Art

Solid-state imaging devices are classified broadly into amplificationtype solid-state imaging devices, which are typically exemplified byCMOS (complementary metal-oxide semiconductor) image sensors, and chargetransfer type imaging devices, which are typified by CCD (charge-coupleddevice) image sensors.

CMOS image sensors have replaced CCD sensors at rapid speed particularlyin the area of portable device-oriented image sensors owning to highperformance and low power consumption characteristics. Such a CMOS imagesensor includes an imaging section having a plurality of pixels arrangedin a two-dimensional array, each of the pixels including a photodiode(PD) serving as a photoelectric conversion element and several pixeltransistors; and peripheral circuits arranged around the imagingsection.

The peripheral circuits include at least column circuits or verticaldriving circuits for transmitting signals in the column direction, andhorizontal circuits or horizontal driving circuits for sequentiallytransferring the signals, which are transmitted column wise by thecolumn circuits, to an output circuit. The pixel transistors areprovided having known configurations such as, for example,four-transistor circuit configuration including transfer, reset,amplifying, and selection transistors; and three-transistor circuitconfiguration including transfer, reset, and amplifying transistorsexcepting the selection transistor.

A CMOS image sensor is generally provided by arranging a plurality ofunit pixels, in which each of the unit pixels includes one photodiodeand several pixel transistors, as a set. However, miniaturization of thepixel size has been notable in recent years. With regard to the CMOSimage sensor including a large number of pixels, many attempts have beendisclosed on CMOS image sensors of the type of sharing pixel transistorswith a plurality of pixels to thereby reduce the number of pixeltransistors.

One of the CMOS image sensors sharing pixel transistors is disclosed,for example, in Japanese Unexamined Patent Application Publication No.Heisei 11 (1999)-331713 which will be given shortly.

On the other hand, another disclosure is made in which the transferefficiency of charges can be increased by suitably devising thestructure of transfer gate in miniaturized design of the pixel. Forexample, disclosed in Japanese Unexamined Patent Application PublicationNo. 2005-129965 (in paragraph 0039 and FIG. 3 therein) is that aphotodiode PD, a floating diffusion (FD) region 101, and a transfertransistor Tr1 as one of the pixel transistors are formed as a part ofpixel, as illustrated in FIG. 1. The transfer transistor Tr1 includes atransfer gate electrode 102 and a channel region 103 formed directlythereunder. Also, in the transfer transistor Tr1, the edge of a transfergate 104, or of the transfer gate electrode 102, toward the photodiodePD, is formed in the shape of convex so that the electric field isgenerated in the photodiode PD toward the transfer gate 104 with moreease. It may be noted in the structure of FIG. 1 that the channel width“a” of the transfer gate 104 on the side of photodiode PD (i.e., thechannel width in contact with the photodiode PD), is larger than thechannel width “b” on the side of floating diffusion (FD) region 101(i.e., the channel width in contact with the floating diffusion (FD)region 101).

SUMMARY OF THE INVENTION

With regard to CMOS image sensor, the gate size of the pixel transistorsincluded in pixels decreases with the decrease in pixel size, and it hasbecome more difficult to maintain the characteristics of the pixeltransistors. As an example, it is difficult for the gate of transfertransistor (hereinafter referred to as transfer gate) for reading outsignal charges from a photodiode PD to a floating diffusion (FD) regionas well, to satisfy both cutoff characteristics and charge transfercharacteristics of the transfer transistor at the same time. That is,the leak current is generated more easily from a photodiode (PD) to afloating diffusion (FD) region when the transfer gate is turned off,while a potential barrier has become difficult to be sufficientlylowered when the transfer gate is turned on for the readout periodbecause of weak channel modulation by the transfer gate.

However, there exist trade-offs between the size of transfer gate andphotoelectric conversion efficiency, in which further increase in thesize of transfer gate within a given pixel area may increase risks, suchas reducing the area of photodiode PD for photoelectric conversion andintercepting some of incident light with the transfer gate when focusinglight.

In this regard, with the structure of transfer gate shown in FIG. 1there may be a risk of reducing the area of photodiode PD since thetransfer gate is formed to protrude toward the photodiode PD. Thisstructure of the transfer gate therefore may reduce the amount ofsaturation charges and cause the interception of incident light.

In light of the foregoing points, it is desirable to provide asolid-state imaging device capable of maintaining transistorcharacteristics of a transfer transistor included in the device yetsecuring a sufficient area of light receiving surface of a photoelectricconversion element even when the pixel size is miniaturized, and also acamera provided with the solid-state imaging device.

A solid-state imaging device according to an embodiment of the inventionincludes a plurality of pixels in an arrangement, each of the pixelsincluding a photoelectric conversion element, pixel transistorsincluding a transfer transistor, and a floating diffusion region. Achannel width of a transfer gate of the transfer transistor on a side ofthe floating diffusion region is formed to be larger than on a side ofthe photoelectric conversion element.

A camera according to an embodiment of the invention includes asolid-state imaging device, an optical system configured to leadincident light to a photoelectric conversion element included in thesolid-state imaging device, and a signal processing circuit configuredto process signals output from the solid-state imaging device. Thesolid-state imaging device includes a plurality of pixels in anarrangement, each of the pixels including a photoelectric conversionelement, pixel transistors including a transfer transistor, and afloating diffusion region. A channel width of a transfer gate of thetransfer transistor on a side of the floating diffusion region is formedto be larger than on a side of the photoelectric conversion element.

In the pixel included in the solid-state imaging device according to anembodiment of the invention, the channel width of the transfer gate ofthe transfer transistor on the side of the floating diffusion region isformed to be larger than on the side of the photoelectric conversionelement. As a result, even when the pixel size is miniaturized, thepotential of a channel region becomes deeper on the side of floatingdiffusion region than on the side of photoelectric conversion elementwhen the transfer gate is turned on during a readout period, and theelectrical field can be generated with more ease in the direction ofcharge transfer. In addition, when the transfer transistor is turnedoff, the leak current from the photoelectric conversion element to thefloating diffusion region is suppressed. Still in addition, since thetransfer gate is formed without protruding its part toward thephotoelectric conversion element, the area of light receiving surface ofthe photoelectric conversion element can be secured.

According to an embodiment of the present invention, the transistorcharacteristics of the transfer transistor can be maintained. Therefore,even when the pixel size is miniaturized, the capability of transferringsignal charges from the photoelectric conversion element to the floatingdiffusion region can be improved. In addition, the area of lightreceiving surface of the photoelectric conversion element can be fullysecured.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiments of the invention will be described in detailwith reference to the following drawings, wherein:

FIG. 1 is a view illustrating main portions of a pixel of the relatedart;

FIG. 2 is a schematic view illustrating a configuration of a solid-stateimaging device according to an embodiment of the present invention;

FIG. 3 is a view illustrating main portions of a first example of apixel according to an embodiment of the present invention;

FIG. 4 is a view illustrating main portions of a second example of apixel according to an embodiment the present invention;

FIG. 5 is a schematic view illustrating a configuration of a solid-stateimaging device according to another embodiment of the present invention;

FIG. 6 is a view illustrating main portions of a third example of apixel according to an embodiment of the present invention;

FIG. 7 is a plan view illustrating an example of a layout of an imagingsection using sharing pixel shown in FIG. 6;

FIG. 8 is a plan view illustrating another example of a layout of theimaging section using the sharing pixel shown in FIG. 6; and

FIG. 9 is a view illustrating a configuration of a camera according toan embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described hereinbelow on a variety ofembodiments by referring to the accompanying drawings. It is notintended to be exhaustive or to limit the invention to those disclosedin the embodiments and illustrated in the drawings.

FIG. 2 is a schematic view illustrating a configuration of a solid-stateimaging device or CMOS image sensor according to an embodiment of thepresent invention. Referring to FIG. 2, a solid-state imaging device 1according to the present embodiment includes an imaging section 3 (i.e.,pixel section) having a plurality of pixels 2 arranged in atwo-dimensional array, and peripheral circuits arranged around theimaging section 3, having a vertical driving unit 4, a horizontaltransfer unit 5, and an output unit 6. Each of the pixels 2 includes aphotodiode PD serving as a photoelectric conversion element and severalpixel transistors (MOS transistors) Tr.

The photodiode PD includes a region configured to accumulate signalcharges generated by the photoelectrical conversion of incident light.The several pixel transistors Tr include four MOS transistors, in thisexample, a transfer transistor Tr1, a reset transistor Tr2, anamplifying transistor Tr3, and a selection transistor Tr4.

The transfer transistor Tr1 serves as a transistor for reading out thesignal charges accumulated in the photodiode PD to a floating diffusion(FD) region which will be described later on. The reset transistor Tr2is a transistor for setting the potential of the floating diffusion (FD)region to a predetermined value. The amplifying transistor Tr3 is atransistor for electrically amplifying the signal charges readout to thefloating diffusion (FD) region. The selection transistor Tr4 is atransistor for selecting a line of pixels and reading out pixel signalstherein to the vertical signal line 8. In addition, although no drawingis provided herein, the pixel may alternatively be formed with thephotodiode PD and three transistors including the transfer, reset, andamplifying transistors excepting the selection transistor Tr4.

In the circuit configuration of pixel 2, the source of the transfertransistor Tr1 is connected to the photodiode PD, and the drain of Tr1is connected to the source of the reset transistor Tr2. The floatingdiffusion (FD) region serving as a charge-voltage conversion unit, whichis arranged between the transfer transistor Tr1 and the reset transistorTr2 (equivalent to the drain region of the transfer transistor Tr1 andthe source region of the reset transistor Tr2), is connected to the gateof the amplifying transistor Tr3. The source of the amplifyingtransistor Tr3 is connected to the drain of the selection transistorTr4. The drains of reset transistor Tr2 and amplifying transistor Tr3are both connected to a source voltage supply unit. In addition, thesource of the selection transistor Tr4 is connected to the verticalsignal line 8.

The vertical driving unit 4 is configured to respectively supply a linereset signal φRST to be applied in common to the gates of the resettransistors Tr2 of the pixels arranged on one line, a line transfersignal φTRG to be applied in common to the gates of the transfertransistors Tr1 of the pixels arranged on one line, and a line selectionsignal φSEL to be applied in common to the gates of the selectiontransistors Tr4 of the pixels arranged on one line.

The horizontal driving unit 5 includes amplifiers or analog-to-digitalconverters (ADC) connected to the vertical signal line 8 of each column,such as analog-to-digital converters 9 in the present example; columnselection circuits (switch units) SW; and a horizontal transfer line 10(bus including the same number of lines as data bit lines, for example).

The output unit 6 includes a further amplifier, or analog-to-digitalconverter and/or signal-processing circuit; such as signal-processingcircuit 11 in the present example for processing outputs from thehorizontal transfer line 10 and an output buffer 12.

The solid-state imaging device 1 is configured for the signals from thepixels 2 on each line to be subjected to analog-to-digital conversionwith each analog-to-digital converter 9, readout to the horizontaltransfer line 10 through the column selection circuits SW which aresequentially selected, and transferred sequentially in the horizontaldirection. Image data readout to the horizontal transfer line 10 aresubsequently output from the output buffer 12 through thesignal-processing circuit 11.

The operation with the pixel 2 is carried out in general as follows.First, by turning on the gates of transfer transistor Tr1 and resettransistor Tr2, the charges in the photodiode PD are all cleared out.Thereafter, the gates of the transfer transistor Tr1 and the resettransistor Tr2 are turned off, and photoelectric charge accumulation iscarried out. Next, immediately before reading photoelectric chargesaccumulated in the photodiode PD, the gate of the reset transistor Tr2is turned on and the potential of the floating diffusion (FD) region isreset. Subsequently, by turning off the gate of the reset transistor Tr2and turning on the gate of the transfer transistor Tr1, respectively,the photoelectric charges from the photodiode PD are transferred to thefloating diffusion (FD) region. The amplifying transistor Tr3electrically amplifies the signal charges upon receiving the charges atthe gate thereof. On the other hand, from the moment of abovementionedreset of the potential of the floating diffusion (FD) region immediatelybefore the charge reading, the selection transistor Tr4 is turned ononly for the pixel 2 to be presently readout. Subsequently, imagesignals subjected to charge-to-voltage conversion and supplied from theamplifying transistor Tr3 included in the presently addressed pixel 2are readout to the vertical signal line 8.

According to the present embodiment, the solid-state imaging device 1includes the transfer gate of the transfer transistor Tr1 configured sothat the transfer of signal charges to the floating diffusion (FD)region can sufficiently be carried out even in the case of miniaturizedpixel size, while fully securing the area of photodiode PD. That is, inthe present embodiment, the channel width of the transfer gate of thetransfer transistor Tr1 on the side of the floating diffusion (FD)region is formed to be larger than on the side of the photodiode PD.Moreover, conversion efficiency can also be improved with thisconfiguration.

FIG. 3 is a view illustrating a first example of a pixel according to anembodiment of the present invention, which includes a photo-diode PD, afloating diffusion (FD) region 20, and a transfer transistor Tr1particularly illustrating its transfer gate 21. In the first exampleillustrated in FIG. 3, a transfer gate electrode 22 constituting thetransfer gate 21 of the transfer transistor Tr1 is arranged at a cornerportion of the square planar photodiode PD, having a convex-shape withits top portion facing a side of the floating diffusion (FD) region 20.

That is, the transfer gate electrode 22 is formed in the shape of neartrapezoid, or triangle with its top portion being removed, so that one(the base) of the sides thereof is adjacent to the side of the squarephotodiode PD intercepted slant by the trapezoid, and approximatelyL-shaped two sides thereof are adjacent to the floating diffusion (FD)region 20. As a result, the photodiode PD of unit pixel illustrated inthe drawing is formed in the shape of pentagon that is resulted from theplanar form of square or rectangle of the photodiode PD by slightly andlinearly taking off one of the corner portions. In addition, thefloating diffusion (FD) region 20 is formed nearly in the planar shapeof the character “L”.

A device isolation region 24 is formed so as to surround the photodiodePD, floating diffusion (FD) region 20, and transfer transistor Tr1, andextends partially under the transfer gate electrode 22. That is, a partof the device isolation region 24 is formed to extend under the transfergate electrode 22 so that the substantial portion of a channel region 23of the transfer gate 21 extends to the side of photodiode PD, having thewidth sufficient to cover the full breadth of the L-shaped floatingdiffusion (FD) region 20.

Although no drawing is provided herein, the photodiode PD is formed inthe present example as a buried-type photodiode PD which includes ann-type semiconductor region (n+ region) used as a charge accumulationregion formed in a p-type semiconductor well region, and a p-typesemiconductor region (p+ region) serving as an accumulation layer formedon the surface side of the n-type semiconductor region. In addition, thefloating diffusion (FD) region 20 as the region equivalent to the drainregion of the transfer transistor Tr1, is formed with the n-typesemiconductor region (n+ region) in this example. Still in addition, thedevice isolation region 24 is formed with the p-type semiconductorregion (p+ region) in this example.

Moreover, in the example, a part of the nearly L-shaped floatingdiffusion (FD) region 20, that is, the portion thereof facing the topportion of the transfer gate electrode 22 (the top of the aforementionedconvex-shape) is formed as the high impurity concentration region havinga small area (i.e., high concentration region: n+ region in thisexample) 26. In addition, other portions of the nearly L-shaped floatingdiffusion (FD) region 20, that is, the portions surrounding the highconcentration region 26 or corresponding to the region between the highconcentration region 26 and device isolation region 24, are formed as aregion 27 having an impurity concentration (i.e., low concentrationregion: n− region in this example) lower than the high concentrationregion 26.

The impurity concentration of the low concentration region 27 is lowerthan the low concentration region in a typical LDD structure, and theregion 27 has an area larger than the low impurity concentration regionautomatically formed in a typical process of PN junction formation.

On the other hand, the high concentration region 26 in the floatingdiffusion (FD) region 20 is shared with the contact region for use inconnecting to pixel transistors. In this example, the impurityconcentration of the high concentration region 26 may reach 1×10²⁰ cm⁻³or higher. In addition, the impurity concentration of the lowconcentration region 27 may be less than 1×10¹⁸ cm⁻³.

According to the first example, since the transfer gate electrode 22 isformed in the shape of nearly trapezoid with a convex top portion facingthe floating diffusion (FD) region 20, the reduction of photodiode PDarea affects only slightly the corner portion of photodiode PD andtherefore the area for the photodiode PD can be secured to remain broad.As a result, even if the pixel is miniaturized, light incident on thephotodiode PD may not be affected by the transfer gate electrode duringlight focusing and the amount of saturation charges can be fullysecured.

In addition, as illustrated in FIG. 3, since the channel width of thetransfer gate 21 on the side of the floating diffusion (FD) region 20 isformed to be larger than on the side of the photodiode PD, cutoffcharacteristics and charge transfer characteristics of the transfertransistor Tr1 can be made compatible, thereby maintaining transistorcharacteristics.

That is, the channel width B on the side of the floating diffusion (FD)region 20 is larger than the channel width A on the side of photodiodePD. This change in the channel width leads to the change in thepotential of the channel region 23, and the electrical field isgenerated owning to the shape effect so that the potential becomesdeeper on the side of the floating diffusion (FD) region 20 than on theside of the photodiode PD when the transfer transistor Tr1 is turned on.For the narrow channel width A, the potential is shallow, while for thewide channel width B, the potential becomes deep. Therefore, thetransfer of signal charges from the photodiode PD to the floatingdiffusion (FD) region 20 can be carried out satisfactorily, and thetransfer capability for signal charges can be improved even the pixelsare miniaturized. In addition, leak current generation is suppressedwhen the transfer transistor Tr1 is turned off.

The reason for the leak current suppression will be described asfollows. In the case where the channel width W is constant throughout,the amount of change in channel potential is the same at both sides onthe photodiode PD and the floating diffusion (FD) region. As a result,when a potential difference is generated so as to apply an electricfield to the channel region for defining the transfer direction with thetransfer gate being turned on, this causes that much amount of potentialdifference even when the transfer gate is turned off.

In contrast, according to the example, since the potential change in thetransfer gate 21 on the side of the photodiode PD is larger than on theside of floating diffusion (FD) region 20, the potential difference atthe time of the transfer gate 21 off can be made small, supposing thechannel potential difference between the transfer gate 21 on the sidesof the photodiode PD and the floating diffusion (FD) region is the sameas the above at the time of the transfer gate on. That is, the channelon the side of the floating diffusion (FD) is closed as compared to theside of the photodiode PD at the time of the transfer gate off, so thatthe leak current can be reduced.

By forming the high concentration region 26 of the floating diffusion(FD) region 20 in common or shared with the contact region, the area ofthe high concentration region can be minimized. In the example, the highconcentration region 26 is not necessary at the locations other than thecontact region. Since the high concentration region is formed byimpurity implantation using a photoresist mask in a typical CMOSprocess, this region is an area larger than the contact area for thecontact region. The device structure is not configured in general sothat the high impurity concentration is formed only in the portion ofthe floating diffusion (FD) region which is in contact with the gate.

On the other hand, when the floating diffusion (FD) region 20 is formedin the shape of L character, the area of the floating diffusion (FD)region 20 is increased. The increase in the area usually causes theincrease in diffusion capacity (i.e., junction capacity) in the floatingdiffusion (FD) region 20 and the decrease in conversion efficiency.

In the example, however, the distribution of impurity concentration issuitably designed so that the n-type high concentration region 26 in thefloating diffusion (FD) region 20 is formed as the part facing theconvex portion of the transfer gate electrode 21, being effective forsubstantially accumulating electric charges and being the contactregion; and that the other part in the floating diffusion (FD) region 20is formed as the n-type low concentration region 27. The junctioncapacity of the low concentration region 27 is quite small. Therefore,the junction capacity of the floating diffusion (FD) region 20 as awhole may not increase considerably and the decrease in conversionefficiency is alleviated.

Signal charges transferred from the photodiode PD to the lowconcentration region 27 having shallow potential in the floatingdiffusion (FD) region 20 are collected to the high concentration region26 having deep potential.

FIG. 4 is a view illustrating a second example of a pixel according toan embodiment of the present invention, which includes a photodiode PD,a floating diffusion (FD) region 20, and a transfer transistor Tr1particularly illustrating its transfer gate 21. In this exampleillustrated in FIG. 4, the transfer transistor Tr1 includes a transfergate 21 formed between a photodiode PD and a floating diffusion (FD)region 20, the channel width of the transfer gate 21 on a side of thefloating diffusion (FD) region 20 is formed larger than on a side of thephotodiode PD.

The photodiode PD is formed in a quadrilateral shape such as a square orrectangle. The floating diffusion (FD) region 20 is formed in the shapeof rectangle, in which one of the sides thereof facing the photodiode PDis in the same length as the opposing side of the photodiode PD. Thetransfer gate 21 includes a rectangular transfer gate electrode 22 and atrapezoidal channel region 23. The channel region 23 is formed as atrapezoid so that the channel width A thereof on the side of photodiodePD is narrower than the width B on the side of the floating diffusion(FD) region 20 and that the channel width of channel region 23 of thetransfer gate 21 is gradually increased from the side on the photodiodePD to the side on the floating diffusion (FD) region 20.

On the other hand, the floating diffusion (FD) region 20 includes a highimpurity concentration region 26 (n+ region in this example) formed atthe center of the rectangular floating diffusion (FD) region 20 in amanner similar to the aforementioned example, and a low concentrationregion 27 (n− region in this example) formed on the remaining portionsof the (FD) region 20. Since the device structure and other featuressuch as impurity concentration and the like are similar to thosedescribed earlier in the first example, the description thereof is notrepeated herewith.

According to the second example, the area of the photodiode PD can besecured to remain broad by forming the photodiode PD in the shape ofsquare, and the amount of saturation charges can be fully secured evenwhen the pixel size is miniaturized. In addition, the electrical fieldis generated in the channel region 23 of the transfer gate 21 so thatthe potential becomes gradually deeper from the side on the photodiodePD toward the side on the floating diffusion (FD) region 20. Therefore,the transfer of signal charges from the photodiode PD to the floatingdiffusion (FD) region 20 can be carried out satisfactorily, and thetransfer capability for signal charges can be improved even when thepixels are miniaturized.

On the other hand, since the floating diffusion (FD) region 20 includesthe high concentration region 26 and the low concentration region 27,the junction capacity of the floating diffusion (FD) region 20 as awhole can remain low and the decrease in conversion efficiency isalleviated.

According to the second example, the signal charges transferred to thelow concentration region 27 of the floating diffusion (FD) region 20 arealso collected to the high concentration region 26. In addition, theeffects similar to those of the first example can be obtained also inthis second example.

The device configuration of the first example illustrated in FIG. 3 issuitably used in the CMOS image sensor in which pixel transistors areshared with a plurality of photodiodes PD. Next, a further embodimentwith regard to such a device configuration will be described.

FIG. 5 is a schematic view illustrating a configuration of a solid-stateimaging device or CMOS image sensor according to another embodiment ofthe present invention. The solid-state imaging device of the presentembodiment is the case where a plurality of sets are arranged, in thatthe set herein is formed including (i) a plurality of pixelsrespectively provided with photodiodes PD as photoelectric conversionelements, i.e., four pixels respectively provided therein with fourphotodiodes PD in this example, and (ii) pixel transistors other thanthe transfer transistor, which are shared with the four photodiodes PDor pixels (i.e., the set being hereinafter referred to as sharingpixel).

Referring to FIG. 5, a solid-state imaging device 31 according to thepresent embodiment includes an imaging section 3 (i.e., pixel section)having a plurality of sharing pixels 32 arranged in a two-dimensionalarray, and peripheral circuits arranged around the imaging section 3,such as a vertical driving unit 4, a horizontal transfer unit 5, and anoutput unit 6. Each of the sharing pixels 32 includes a plurality ofphotodiodes PD serving as photoelectric conversion elements, i.e., fourphotodiodes PD in this example, four transfer transistors, one resettransistor, one amplifying transistor, and one selection transistor.That is, the pixel transistors other than the transfer transistor suchas the reset, amplifying, and selection transistors are shared with thefour photodiodes PD, as mentioned above.

In the circuit configuration of the sharing pixel 32, as shown in FIG.5, these four photodiodes PD1, PD2, PD3, and PD4 are connected to thesources of corresponding four transfer transistors Tr11, Tr12, Tr13, andTr14, respectively, and the drains of the four transfer transistorsTr11, Tr12, Tr13, and Tr14 are connected to the source of one resettransistor Tr2. The common floating diffusion (FD) region serving ascharge-voltage conversion unit formed between the transfer transistorsTr11, Tr12, Tr13, and Tr14 and the reset transistor Tr2 is connected tothe gate of the one amplifying transistor Tr3. The source of theamplifying transistor Tr3 is connected to the drain of the one selectiontransistor Tr4. The drains of the reset transistor Tr2 and theamplifying transistor Tr3 are both connected to a source voltage supplyunit. In addition, the source of the selection transistor Tr4 isconnected to the vertical signal line 8.

To the gates of the transfer transistors Tr11, Tr12, Tr13, and Tr14,line transfer signals φTRG1, φTRG2, φTRG3, and φTRG4 are applied,respectively. A line reset signal φRST is applied to the gate of thereset transistor Tr2, and a line selection signal φSEL is applied to thegate of the selection transistor Tr4.

Since the configuration of the vertical driving unit 4, horizontaltransfer unit 5, output unit 6 and the like are similar to thosedescribed earlier referring to FIG. 2, the description thereof is notrepeated herewith.

FIG. 6 is a view illustrating the planar configuration of the sharingpixel 32 of a third example according to another embodiment of theinvention. A set of the sharing pixel 32 according to this example usesthe aforementioned pixel structure as shown in FIG. 3 and includes fourpixels in a two-by-two pixel sharing configuration arranged with two ofthem horizontally and two of them vertically.

In this example, as shown in FIG. 6, the common floating diffusion (FD)region 20 is arranged at the center of the structure so that thefloating diffusion (FD) regions 20 may be shared with each other. So asto hold the common floating diffusion (FD) region 20 in the middle ofthe structure, four pixels (each having the pixel structure shown inFIG. 3) are arranged horizontally and vertically to be point symmetriccentering on corner portions 211, 212, 213, and 214 on the side of thetransfer gate 21. Therefore, the floating diffusion (FD) region 20 atthe center is formed in the planar shape of cross having a highconcentration region 26 at its center and low concentration regions 27each on the arm portions of the cross. In addition, in regard to thecontact with the floating diffusion (FD) region 20, the device isolationregion 24 for isolating respective photodiodes PD1, PD2, PD3, and PD4 isbrought to be in contact only with the top portion of the lowconcentration regions 27 on the arm portion.

Since other features are similar to those described earlier withreference to FIG. 3, the description thereof is not repeated herewith.

With the configuration of sharing pixel according to the third example,by disposing the four pixels to be point symmetric centering on thefloating diffusion (FD) region 20, namely, on the corner portions of thetransfer gate 21, it becomes possible to densely arrange pixels in theimaging section 3 in which a large number of pixels are mounted as willbe described later on. Also, according to the example, since the channelwidth of the transfer gate 21 increases from the side on the photodiodePD toward the side on the floating diffusion (FD) region 20 and thechannel potential changes accordingly owning to the shape effect of thetransfer gate 21 in a manner similar to the aforementioned example, thetransfer efficiency of signal charges is improved.

Moreover, since the common floating diffusion (FD) region 20 is formedin the shape of cross having its central portion as the highconcentration region 26 and the other portions thereof as the lowconcentration regions 27, the junction capacity of the floatingdiffusion (FD) region 20 decreases considerably, the charge-voltageconversion efficiency is improved, or the decrease in the conversionefficiency is reduced. Particularly, the portion of contact between then-type floating diffusion (FD) region 20 and the p-type device isolationregions 24 is only the arm edge portion formed as the cross shaped lowconcentration region 27, the junction capacity between the floatingdiffusion (FD) region 20 and the device isolation regions 24 is furtherdecreased, and the conversion efficiency is improved accordingly. Inaddition, similar effects to those of the first example are alsoobtained in the present embodiment.

FIG. 7 is a schematic view illustrating a layout of the imaging section3 using the sharing pixel 32 shown in FIG. 6. This example illustratesthe layout with a plurality of sharing pixels 32 arranged in a squarearray. That is, in the layout of the example, a set of reset transistorTr2, amplifying transistor Tr3, and selection transistor Tr4 arearranged on one of the sides of respective sharing pixels 32 in thevertical direction, or on the bottom side in this example. A pluralityof such structure of sharing pixels 32 are arranged horizontally andvertically as an orthogonal coordinate array system.

The reset transistor Tr2 includes a source region 41, a drain region 42,and a reset gate electrode 43. The amplifying transistor Tr3 includes asource region 44, a drain region 45, and an amplifying gate electrode46. The selection transistor Tr4 includes a source region 47, a drainregion 44, and a selection gate electrode 48. In providing the abovestructure, the source region 44 of the amplifying transistor Tr3 and thedrain region 44 of the selection transistor Tr4 are formed in common andshared with each other.

In addition, the high concentration region 26 of the floating diffusion(FD) region 20, the source region 41 of the reset transistor Tr2, andthe amplifying gate electrode 46 of the amplifying transistor Tr3 areconnected with each other via wiring 49. Moreover, the source region 47of the selection transistor Tr4 and a vertical signal line 8 areconnected via wiring 50.

According to the layout of the imaging section shown in FIG. 7, a largenumber of sharing pixels 32 are able to be arranged densely horizontallyas well as vertically, whereby high-resolution solid-state imagingdevices can be provided.

FIG. 8 is a schematic view illustrating a further layout of the imagingsection 3 using the sharing pixel 32 shown in FIG. 6 according toanother example. This example illustrates the layout with a plurality ofsharing pixels 32 arranged aslant (or in the honeycomb structure). Thatis, in this example, in a manner similar to the structure of FIG. 6, aset of reset transistor Tr2, amplifying transistor Tr3, and selectiontransistor Tr4 are arranged on one of the sides of respective sharingpixels 32 in the vertical direction, or on the bottom side in thisexample.

Such structures of the sharing pixels 32 are arranged horizontally andvertically as an orthogonal coordinate array with two orthogonal axesthereof inclined with respect to horizontal and vertical directions,respectively. In the example illustrated in FIG. 8, the sharing pixels32 are arranged as an array on the orthogonal coordinate system with thetwo orthogonal axes inclined at 45 degrees with respect to horizontaland vertical directions, respectively.

Since other features of sharing pixel structure are similar to thosedescribed earlier with reference to FIG. 7, the portions similar tothose in FIG. 7 are shown with identical representations and thedescription thereof is not repeated herewith.

According to the layout of the imaging section of FIG. 8, a large numberof sharing pixels 32 are able to be densely arranged, whereby asolid-state imaging device can be provided having a resolution higherthan the imaging section of FIG. 7.

FIG. 9 is a schematic view illustrating the configuration of a cameraincluding the abovementioned CMOS image sensors according to anembodiment of the present invention. Referring to FIG. 9, a camera 40according to the present embodiment includes an optical system (opticallens) 41, a CMOS solid-state imaging device 42, and a signal processingcircuit 43. As to the solid-state imaging device 42, any one of thepixel configurations described in the aforementioned first through thirdexamples, and preferably one detailed in the first or third example withthe device layout of either FIG. 7 or FIG. 8 may be used. The cameraaccording to the present embodiment may also include a camera moduleformed by modularizing the optical system (optical lens) 41, the CMOSsolid-state imaging device 42, and the signal processing circuit 43.

The optical system 41 is configured to carry out image formation on theimaging surface of the CMOS solid-state imaging device 42 with the imagelight (incident light) from the subject. Subsequently, the incidentlight is converted to signal charges in response to the amount of theincident light with photoelectric conversion element (light receivingunit) of CMOS solid-state imaging device 42, and the signal charges areaccumulated for a fixed period of time in the photoelectric conversionelement. The signal processing circuit 43 is configured to carry out avariety of signal processing on the signals output from CMOS solid-stateimaging device 42, and to subsequently output resulted image signals.

With the camera according to an embodiment of the invention, the amountof saturation charges and conversion efficiency are maintained, and thetransfer of signal charges to the floating diffusion region is improvedeven when pixel size is miniaturized, whereby high-resolution camerascan be provided.

According to an embodiment of the present invention, various electronicdevices including the abovementioned camera shown in FIG. 9 or thecamera module can be provided.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

1. (canceled)
 2. An imaging device, comprising: a plurality ofphotoelectric conversion elements including a first photoelectricconversion element, a second photoelectric conversion element, a thirdphotoelectric conversion element, and a fourth photoelectric conversionelement; a plurality of pixel transistors including a transfertransistor, a reset transistor, an amplifier transistor, and a selectiontransistor; and a floating diffusion region; wherein, a channel width ofa transfer gate of the transfer transistor is larger along a side facingthe floating diffusion region than a width of the transfer gate along aside facing the photoelectric conversion element, and wherein thefloating diffusion region, the reset transistor, the amplifiertransistor, and the selection transistor are shared with the pluralityof photoelectric conversion elements; the first and second photoelectricconversion elements are disposed in a first row, the third and fourthphotoelectric conversion elements are disposed in a second row, thereset transistor, the amplifier transistor, and the selection transistorare disposed in a third row, and the second row is disposed between thefirst row and the third row.
 3. The imaging device according to claim 2,wherein a high concentration region is formed partly in the floatingdiffusion region.
 4. The imaging device according to claim 3, whereinanother portion of the floating diffusion region around the highconcentration region and between the high concentration region and anelement isolation region is formed as a low concentration region havinga lower impurity concentration than the high concentration region. 5.The imaging device according to claim 3, wherein the high concentrationregion in the floating diffusion region is also used as a contactregion.
 6. The imaging device according to claim 3, wherein the highconcentration region is adjacent to a top portion of a convex shapeportion of the transfer gate.
 7. The imaging device according to claim3, wherein a portion is included in which a length between end portionsof the floating diffusion region other than the position where thefloating diffusion region is associated with the transfer gate graduallyincreases from the straight line connecting between the both endportions toward the side of the floating diffusion region.
 8. Theimaging device according to claim 2, wherein image signals from theplurality of pixel transistors are readout to a vertical signal line. 9.The imaging device according to claim 8, further comprising: ahorizontal transfer portion connected to the vertical signal line,wherein the image signals are readout from the vertical signal line tothe horizontal transfer portion.
 10. The imaging device according toclaim 9, wherein the horizontal transfer portion includes at least oneselected from an amplifier, an analog/digital converter, a columnselection circuit, and a horizontal transfer line.
 11. The imagingdevice according to claim 2, wherein, under the transfer gate, a channelportion is included in a region surrounded by the photoelectricconversion element, the floating diffusion region, and the elementisolation region.
 12. The imaging device according to claim 11, wherein,under the transfer gate, a length of a portion where the photoelectricconversion element is connected to the channel portion is a channelwidth on a side of the photoelectric conversion element, and a length ofa portion where the floating diffusion region is connected to thechannel portion is a channel width on a side of the floating diffusionregion.
 13. The imaging device according to claim 12, wherein thefloating diffusion region, the photoelectric conversion element, and theelement isolation region around the channel portion are formed with asemiconductor layer having a conductivity type opposite to aconductivity type of the floating diffusion region.
 14. An apparatuscomprising: an imaging device; an optical system configured to leadincident light to at least one of a plurality of photoelectricconversion elements in the imaging device; and a signal processingcircuit configured to process an output signal of the imaging device;wherein the imaging device includes: a plurality of pixels in anarrangement, each of the pixels including at least one of the pluralityof photoelectric conversion elements and at least one of a plurality ofpixel transistors; the plurality of photoelectric conversion elementsinclude a first photoelectric conversion element, a second photoelectricconversion element, a third photoelectric conversion element, and afourth photoelectric conversion element; the plurality of pixeltransistors include a transfer transistor, a reset transistor, anamplifier transistor, and a selection transistor; and a floatingdiffusion region; wherein, a channel width of a transfer gate of thetransfer transistor is larger along a side facing the floating diffusionregion than a width of the transfer gate along a side facing thephotoelectric conversion element, and wherein the floating diffusionregion, the reset transistor, the amplifier transistor, and theselection transistor are shared with the plurality of photoelectricconversion elements; the first and second photoelectric conversionelements are disposed in a first row, the third and fourth photoelectricconversion elements are disposed in a second row, the reset transistor,the amplifier transistor, and the selection transistor are disposed in athird row, and the second row is disposed between the first row and thethird row.
 15. The apparatus according to claim 14, wherein a highconcentration region is formed partly in the floating diffusion region.16. The apparatus according to claim 15, wherein another portion of thefloating diffusion region around the high concentration region andbetween the high concentration region and an element isolation region isformed as a low concentration region having a lower impurityconcentration than the high concentration region.
 17. The apparatusaccording to claim 15, wherein the high concentration region in thefloating diffusion region is also used as a contact region.
 18. Theapparatus according to claim 15, wherein the high concentration regionis adjacent to a top portion of a convex shape portion of the transfergate.
 19. The apparatus according to claim 15, wherein a portion isincluded in which a length between end portions of the floatingdiffusion region other than the position where the floating diffusionregion is associated with the transfer gate gradually increases from thestraight line connecting between the both end portions toward the sideof the floating diffusion region.
 20. The apparatus according to claim14, wherein image signals from the plurality of pixel transistors arereadout to a vertical signal line.
 21. The apparatus according to claim20, further comprising: a horizontal transfer portion connected to thevertical signal line, wherein the image signals are readout from thevertical signal line to the horizontal transfer portion.
 22. Theapparatus according to claim 21, wherein the horizontal transfer portionincludes at least one selected from an amplifier, an analog/digitalconverter, a column selection circuit, and a horizontal transfer line.23. The apparatus according to claim 14, wherein, under the transfergate, a channel portion is included in a region surrounded by thephotoelectric conversion element, the floating diffusion region, and theelement isolation region.
 24. The apparatus according to claim 23,wherein, under the transfer gate, a length of a portion where thephotoelectric conversion element is connected to the channel portion isa channel width on a side of the photoelectric conversion element, and alength of a portion where the floating diffusion region is connected tothe channel portion is a channel width on a side of the floatingdiffusion region.
 25. The apparatus according to claim 24, wherein thefloating diffusion region, the photoelectric conversion element, and theelement isolation region around the channel portion are formed with asemiconductor layer having a conductivity type opposite to aconductivity type of the floating diffusion region.